Electric discharge device



Sept. 18, 1951 E. B. NOEL mzcmzc DISCHARGE m-zvrca:

Filed 001:. 29, 1948 lnven-kor: Edward B. NoeL, b M6. K fi His AT-tor'neg.

Patented Sept. 18, 1951 atec'rnlc mscnanca nsvrcs Edward 1:. Noel, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New York Application October 29,1948, Serial No. 51,164

' Claims. (CL 313-221) This invention relates to electrical discharge devices of the type discloud in my co-pending application (now abandoned) Serial No. 560,775, filed October 28, 1944 and owned by the General Electric Company, the assignee of the present application. This application is a continuationin-part of my aforesaid co-pending application. As disclosed in my co-pending application, electrical discharse devices are known which have envelopes or internal parts of fused or vitreous silica or quartz (810:) that tend to devitrify during operation, thus becoming granular, frosty, or

cloudy in'appearance, and more or less opaque to the desired radiation from inside the envelope. Common examples of such devices are discharge lamps of the positive column type in which a discharge between electrodes suitably spaced apart is constricted by pressure of the operating atmosphere into a narrow cord or thread, that is recognizably distinct from the characteristic phenomena closely associated with the electrodes themselves: e. g., the lamps marketed under the 2 alkaline earth oxldes,'including those of barium and strontium, though it also occurs in devices having unactivated" refractory metal electrodes, such as those disclosed in U. S. Patent No. 2,353,668 to Donald D. Hinman. Unlike the chemical attack and discoloration of glass envelopes by alkali metal vapor that is mentioned in U. S. Patent No. 2,238,777 to Lemmers and Goodwin, devitriflcation is a mere change of structure in the silica, itself, from amorphous to crystalline.

Quite surprisingly, in view of this radical differencebetween the devitrification of quartz and the chemical attack and discoloration against which glasses have heretofore been protected by glazes as in Patent No. 2,238,777, I have discovered that quartz devitriflcation can be retarded and largely prevented by means of glazes applied to the quartz surface which is so highly heated that it would normally tend to devitrify. Quite obviously, the mechanism of inhibition must needs be very diilferent in the two cases as is designations C-HS, All-4, FHA, Ali-5, whose higher. The operating atmosphere may consist of ionizable gas 'or vapor. or both-such as one or more of the rare inert monatomic gases like argon, xenon, etc., or one or more vaporlzable metals like mercury. cadm1um,zinc, or thallium,

eration. or may be so much less than this that the device operates with an unsaturated atmosphere. When vaporizable substances are used. rare gases may also be included to assist in starting the discharge.

Fused quartz or quartz glass is about the most refractory light and radiation-transmitting material that is practicable for discharge envelopes, but it has the drawback of increasingly rapid devitriflcation at temperatures well below the softening point of quartz, about 1400 C. At temperatures around 900 0., more or less, devitriflcation becomes rapid enough to be a factor in limiting the useful life of the lamp. Devitrification is more rapid in devices whose electrodes are activated with somewhat volatile oxides like the etc. The amount of such substance(s) in a lamp may either exceed what will vaporize durin opshown conclusively from the fact that devitrification of the highly heated inner surface of a quartz envelope containing a gaseous atmosphere chemicallyinert to quartz, such as rare gas and mercury vapor, is also inhibited when such a glaze is applied to the outer surface only of the envelope, the inner surface of which is left uncoated. Obviously. coating the outer surface of a glass or a quartz envelope containing an ionizable atmosphere chemically active with respect to glass or quartz would be ineffective for protecting the inner surface of such envelope from chemical attack by the gaseous atmosphere.

As disclosed in my co-pending application Serial No. 17,423, filed March 2'7, 1948, some commercial mercury vapor discharge lamps having quartz envelopes enclosed in sealed glass jack-- ets filled with nitrogen and operated with the quartz envelope at elevated temperatures were envelope and, a gradual increase in the starting voltage of the lamp to terminate its useful life prematurely. By constructing the lamp to operinterior of the quartz envelope. The lower tem- I perature of the jacket reduced the amount of hydrogen in the Jacket and the lower temperature of the quartz envelope reduced the rate of diffusion of hydrogen through the quartz. The starting voltage of the lamps remained constant and the inner surface of the envelope remained clear and unfrosted for a useful life of 5,000 hours and more under the same test conditions as the prior commercial lamps.

The application of a glaze to either the inner surface of a quartz envelope, as disclosed in my co-pending application Serial No. 560,775, or the 4 outer surface thereof, as additionally disclosed in the present application, is effective for preventing devitriflcation of the quartz because, I now a definite advantage for the lamps which had envelopes glazed .on the outersurface.

The principal requirements of an efl'ective glaze on the inner envelope surface are adhesion and permanence; it must not quickly crack or flake 011, or be otherwise removed, destroyed, or

deteriorated under exposure to the discharge and the high temperature which this entails. The laze must also be sufliciently permeable to or transmissive of the light or other desired radiation from the interior of the envelope, and should not seriously affect or react with the electrodes or other internal parts of the device, or with the be produced with mere fluxing material such. as

B203 and P205 (used either singly or together), .without any higher melting material except the silica of the quartz article or part itself; onthe other hand, stable and adherent glazes have al been formed on' quartz with relatively high-melting oxides, such as alumina, thoria, zirconia,

beryllia, and titanium dioxide, without the aid of any lower-melting flux. In general, nevertheless, I have been able to glaze quartz articles more easily, uniformly, and effectively when fluxing componentslike boric and phosphoric oxides are associated with higher melting oxides such as those just mentioned. Compounds of the alkali or alkaline earth metals should be avoided.

The coefficient of expansion of silicav is so low (only about 5x10 that'itis difllcult or impracticable .to match it in a glaze. although such glazes may have relatively low expansion coefficients, such as l'5x-10-7, as compared with some 90 x for ordinary glasses. In this'connection,

' it is to beremarked that while both alumina and discharge atmosphere. A considerable variety of v vitreous glazes have been found to answer, or can.

be modified to do so, including various known glazes such as the boric oxide types mentioned in the Lemmers and Goodwin Patent No. 2,238,777, or the silica, boric oxide, and alumina glasses that are'known commercially as 790-,H glass," marketed by Corning Glass Company, and 376-F flux glass, for which the formula is hereinafter stated. Many of the glazes that prevent diffusion of hydrogen through the hot quartz, and thus devitriflcatlon of quartz, will also protect it from chemical attack by metals which might .be used for the operating atmosphere of a'discharge device, such as cadmium or zinc, or their amalgams.

To facilitate the glazing and to unite the glaze more homogeneously with the underlying quartz, it is desirable to use material which fluxes or added silica stabilize 320: or P205, alumina tends to raise the thermal expansion ooeflicient of the glaze. By making the glaze coating or filmsufficiently thin, not only can flaking or cracking of! of the film be avoided, but evenvthe superficial cracking that is commonly. known as'crazing can be obviated; and yet such a thin film can still bequite effective against troublesome devitriflcation. Even though crazing of the glaze may not necessarily make'it worthless, it naturally ren-. ders the device suspect, since it is always possible that the crazed film may flake ofl in service, or that the cracks may so far impair its continuity as to render it ineffective against devitriflcation, or even that the cracks may extend themselves into the quartz wall under the-stress of high internal pressure in the device, and ultimately cause other oxides as glaze materials or components,

Also, chemical combinations between or among.

unites with the quartz or silica in a glass-forming sense. I prefer oxides of elements that are close to silicon in the Periodic System, and especially boric and phosphoric oxides (B103 and P205), which fuse and unite with the quartz at temperatures lower than its fusing orsoftening temperature. With such fluxing material may be associated other material'that tends to stabilize the glaze coating or films against fusion or vaporization, -or against absorption of atmospheric moisture. One preferred material of this sential. On the one hand, very good glazes can this refers to the composition of the flnal glaze on the article, andnot to the form in which such materials are embodied as batch ingredients.

Neither is any implication intended as to whether the glaze materials do or do not enter into chemical combination withone another, or with the underlying quartz of the glazed article. In some cases; the union is probably vno more intimate than a condition'oi solid solution; in others, silicates or other such combinations may be formed.

glaze components may be: used as batch materials,

such as BaOzfPzOs or SiOaPrO5 In general, the

use'of phosphates in connection with borates has the advantage that the B20: is considerably stabilized in the glaze, and the latter is not strikingly hygroscopic like mere boric oxide.

To illustrate the possible .variety of batch ingredients, it may be pointed out that boric oxide or in volatile base salts like the ammonium phosphate (NH!) 2HPO4, or as other phosphates silica in flnely divided pure commercial forms, or as a silicic acid like HzSiOa. 'Borates or phosphates which do ,not break down may also be used for the same purposes as P205; and there is some question whether in practice boron'nitride or titanium nitride do or do not break down and pheric moisture.

- alkaline earth metals should generally be avoided.

In general, the raw or batch materials may be any which at the heat which they undergo lose all constituents except such as form desired or unobiectionable glaze ingredients.

A convenient method of applying the glaz materialis by deposition over the inside of a quartz tube from a carrier liquid, such as acetone, for

example; very much as fluorescent lamp tubes are commonly coated with phosphors. For this purpose the tube may be held upright and filled with the liquid mixture from the bottom, and the mixture then drained out. After drying out of the liquidfrom the deposited coating, the coating may be "fired in" by externally heating the tube. When this is done with the internal tube surface at asumciently high temperature (preferably at or close to the softening temperature of the quartz), the glaze materials interfuse into and unite with the silica in such a waythat in the subsequent operation of the tube they do not readily evaporate and leave the silica unprotected,,even though the tube wall temperature .may

in service exceed the temperatures at which P205 and 1320: show substantial vapor pressures. These temperatures are for P205 well below its sublimation temperature of about 250, and for B203 well below its boiling point of about 1500' C. Afterward the current leads to the electrodes may be fused and sealed intothe tube ends in the usual may without destroying the glaze film, which covers the ends of the envelope cavity thus formed. End plugs or pieces of seal glass can be sealed into or to the tube ends as usual without injuring the glaze onthe tube. Such 'end pieces are cooled by the current inleads extending.-

through them to tneelectrodes in the envelope, and are not in practice subject to devitriflcation, or to attack by metals like cadmium or zinc.

The formula of one glaze which has given very good protection against devitriflcation is as fol- Whatever the glaze batch used, care is taken not to coat the envelope too heavily and thus produce too thick a glaze film on the finished tube.

To bake or fire-in the glaze, the envelope tube I may be heated externally with an oxy-hydrogen flame. which may be passed back and forth and shifted around to play on the tube from every direction. This may done with the tube exposed to the atmosphere, and before sealing into its ends the current leads to the electrodes, or the,end pieces of seal glass that surround the leads; In this way, the tube wall may be heated nearly or approximately to the softening temperature of the quartz, thus expelling all unwanted gaseous or vaporizable matter from any thermally decomposable ingredients of. the glass batch, and uniting or combining the residues with the silica at the inner surface of the tube, as already explained. -When finely divided silica is included in the components of the glaze batch, the flux (e. 8., 3:0: or P205) combines or interfuses with these finely divided particles of silica rather more readily than with the solid inner surface of the tube, so that a thicker coat or film of interfused and thus remain to vaporizeafter the tube has lows:

Boric acid, mac; g 31.2 Precipitated silica g 15.6 Acetone cc 235 V The fired-in boric oxide and silica coating resulting from. this formula is extremely hygroscopic, so that it is necessary to make the tube up into a lamp and complete and seal off the latter at once, or to store it under effective precautions against absorption of moisture. This'is because the boric oxide would in a short time be converted to'boric acid by the moisture of the air, and the water vapor evolving from this boric acid during the exhaust-processing of the device (or after it had been sealed oil and put in service) would spoil the device.

The formula of another good glaze which has proved more stable is as follows:

Precipitated silica e g 15.6

Diammonium phosphate, (NHelaHPOe 34.4 Acetone M 235 85% orthophosphoric acid, HJPO4 cc 15 The fired-in coating resulting from this formula is much less hygroscopic, and need not be used so promptly or protected so completely from atmos- Another good glaze formula islthat of the 376-1 flux glass above mentioned:

been-completed and put in operation. Accordingly, any B20: or P205 that is not stabilized or. held by the silica of the tube wall or by a stabilizing glaze component is gotten rid of.

As already mentioned, careshould be taken notto have too thick a glaze coat or film on the interiorof the tube. Ordinarily, the glaze batch formulas given result in a glaze of proper thickness. If the glaze film shows crazing, it should bethinner; if the tube shows little improvement over an unglaze'd one as regards devitriflcation during operation, the glaze needs to be thicker. In the case of a lamp containing only gas or mercury, a good glaze satisfactorily applied can be expected to allow operation three or four times as long as without glaze before devitriflcation becomes serious. In some cases,

internally glazed tubes start less easily than unglazed ones, requiring a higher break-down voltage Another method of internally glazing tubes X is to' pass through the tube an atmosphere'comprising suitable glaze constituents, and to heat the tube externalLv to cause these constituents to react, unite, or flux with the inner quartz surface. For example; methyl phosphate produced from methanol and phosphoric acid can be in mixed with air and burned in the tube to glaze it internallywith PzOs; or methyl borate can be similarly produced and applied to coat the tube with B203. This method can also be employed to glaze internally a lamp bulb that has been completed except for sealing in the electrodes. etc.

Instead of' internally glazing a lamp bulb or tube. a hollow quartz slug can be internally glazed before being drawn out into tubing, the adherent glaze stretching with the quartz so that the tubing carries a uniform internal film.

Such glazing may be done by any of the meth- 7 ods hereinbefore described. In this case, the glaze inside the slug may be thicker than would bedesirableinthelammortheglazeinthelamp 1 iiiay be could easily be made if the glaze were first applied to the actual lamp tube. After drawing to the desired bore and-wall thickness, the tubing may be cut up into suitable lamp tube lengths and made into lamps.

In the drawing. F g. -1 shows in longitudinal section one form of discharge device embodying the invention; Fig. 2 shows an envelope fora discharge device also embodying the invention; 7

and Fig. 3 is a front elevational view of a lamp shaving a discharge device embodying my invention.

' The discharge device I illustrated in Fig. 1

has an envelope 2 made upof a quartz tubular wall portion having hemispherical domes or caps I of borosilicate seal glass hermetically fused to its ends. The maintungsten inleads l for the main dischargesuppcrting electrodes 5 and the inlead I for the auxiliary starting electrode are provided with seal glass I hermetically united therewith and with the glass caps 3 to seal said inleads 4 through the caps 3. Seals of this kind are disclosed and claimed in the U. 8. Patent 2,177,685, issued October 31, 1939 to l3ol et al. and to the General Electric-Company. Such seal glass is not subject to devitr'ification or to chemical attack by hot ionized metal vapors like zinc or cadmium, nor is the glass of caps 3.

A supply of mercury which vaporizes completely at an envelope temperature slightly lower than the operating temperature of the envelope is indicated by a droplet 8. The device thus operates with an unsaturated or superheated atmosphere of mercury vapor as disclosed and claimed in the U. 8. Patent 2,247,176, issued June 24, 1941 to Pirani et al. and assigned to the Gensurface of the quartz tubular portion I! and overlaps the fused joints between the endsof the Dortion II and the glasscaps i. The glaze I: on the outer surface of the quartz is also eflective for preventing thc-difiuslon of hydrogen through the hot quartz and thus increases the useful life of the lamp incorporating the are tube.

The external glaze may be applied conveniently by dipping theenvelope ll held in a horizontal position with the end caps I but notthe electrode inleads mounted thereon into a thin suspension oi" milled boric anhydr'ide (B203) in'butyl acetate and rotating the envelope to cover the area indicated in the drawing. The coated tube is then heated with an pxy-hydrogen flame to' near the working temperature of quartz. The resulting glaze does not craze and is not hygroscopic eral Electric Company. The main discharge sup-.

porting electrodes 5 include .tungsten wire coils welded around the inner ends of the two inleads I, while the inner end of the extra tungsten in'lead i serves as a starting electrode. The

exhaust tip 9 is also shown through which the envelope is exhausted of air and the droplet of mercury, together with the usual starting gas,

such as argon. at a pressure of a few mm. .is introduced into the envelope during manufacture'of the device. The electrodes 5 also include the usual material ofhigher electron emissivity than tungsten, such as'barium and strontium oxide or thorium metal. The quartz tubular portion of theenvelope 2 is internally glazed with a vitreous film Ill (whose thickness is greatly exaggerated in the drawing) which is not involved in the seals between the ends of the quartz tube and the glass caps 3.

To obtain good eflicie'ncy of lightoutput. I prefer to use a current input for the device such the envelope and resulted in short-lived lamps with devitrifled inner quartz surfaces. As pointed out above. the glaze on the quartz prevents the diffusion oi! hydrogen therethrou'gh to substantially increase the useful operating life of such lamps. a

In Fig. 2 of the drawing, '1 have shown an envelope ll similar to the envelope 2 shown in Fig. 1 except that a glaze I2 is applied to the outer if properly applied.

After the glaze has been applied. the manufacture of the device is completed in the usual manner by mounting the electrodes in the envelope, through the openings in the glass caps 3, exhausting and then filling the envelope through the usual exhaust tube with a discharge-conducting atmosphere.

The devices described above may be incorporated in lamps having glass jackets, such. as that shown in Fig. 3 which is of the type disclosed 'in my co-pending application gerial No. 17,423

mentioned above. In a'specifi'c lamp of the structure shown in Fig. 3 and having a 1000watt'rating, the en-I velope ll of the device had an inner diameter of 25 millimeters and a gap of 150 millimeters be-' tween its main electrodes. The enlarged center portion of the jacket II had an outer diameter oi.v 3 inches of 88.9 millimeters and a length of approximately 8 inches. The jacket ll was first exhausted and then filled with nitrogen at V: at-

mosphere pressure and consisted of 172 glass having the following composition:

Per cent 1 SiO: 56.4

A120; 20.8 NazO 1.6 B20: 1 4.4 CaO 4.9 M 1 1.9

The outer surface of the quartz portion of the envelope ll of the device had an external glaze thereon applied as described in connection with Fig. 2. q

By using the glaze on the outer surface of the envelope the time for exhausting the glass jacket was considerably reduced without hydrogen appearing in the envelope for a long useful life of the lamp. 3 s

As shown in Fig. 3 of the drawing, the lamp comprises a sealed outer glass jacket ll having a screw type base It and a stem tube It at one end. The dischargedevice provided with a glaze on the outer surface of the quartz portion of its envelope II in accordance with invention is supported in the jacket ll by a U-shaped wire 'mount II; The mount has bridging members [8 engaging the envelope i I and is welded at one end to an inlead l 9 passingthrough the stem l6 and electrically connected at its other end to theupper electrode of the device. A pair of spring braces 20 bearing against the jacket I4 and attached.

to opposite ends ofthem'ount II are provided to strengthen the latter. The starting electrode at the lower end of the device is electrically con nected to the mount I! through a resistance 2| in the art that the'glaze on the quartz. portion oi the envelopes 01' high pressure discharge devices making the quartz impervious to hydrogen at temperatures at which hydrogen difluses through unglazed quartz makes possible a reduction in the size of the envelope having such a quartz portion to lower the cost of a device of given wattage withoutshortening the useful operating life of the device;

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A gaseous electric discharge device deleteriously afiected by hydrogen in its discharge conducting gaseous atmosphere comprising a sealed envelope containing a discharge conducting gaseous atmosphere free from hydrogen and having a quartz wall portion, said device being designed for operation with the said quartz wall portion at a temperature at which quartz is pervious to hydrogen and an alkali and alkaline earth tree glaze consisting essentially of-material oi the group consisting of boric oxide and phosphoric oxide on the external surface of said quartz wall portion making it imperviousto hydrogen at said temperature to keep the gaseous atmosphere within said envelope free from casual hydrogen "in the atmosphere without said envelope.

2. A gaseous electric discharge device as in claim 1 wherein the device comprises also a sealed vitreous jacket enclosing the sealed envelope.

ing a quartz wall portion, said device being de- 45 signed for operation with the said quartz wall portion at a temperature at which quartz is pervious to hydrogen, said atmosphere being chemically inert to quartz at said temperature, and an internal alkali and alkaline earth free glaze consisting essentially of material of the group consisting of boric oxide and phosphoric oxide on said quartz wall portion making it impervious to hydrogen at said temperature to keep the gaseous atmosphere within said envelope free from casual hydrogen in theatmosphere without said envelope.

4. A gaseous electric discharge device deleteri-- ously aiiected by hydrogen in its discharge conducting gaseous atmosphere comprising a sealed envelope containing a discharge conducting gaseous atmosphere free from hydrogen and being made up of a quartz tube closed at each end by glass caps, said device being designed for operation with the quartz tube at a temperature at which quartz is pervious to hydrogen, said atmosphere being chemically inert to quartz at said temperature, and a vitreous alkali and alkaline earth free glaze consisting essentially of material of the group consisting of boric oxide and phosphoric oxide on said tube making it impervious to hydrogen at said temperature to keep the gaseous atmosphere within said envelope free from casual hydrogen in the atmosphere without said envelope.

. 10 I 5. A gaseous electric discharge 'devicedeleteriously ailected by hydrogen inits discharge conducting gaseous atmosphere comprising a sealed envelope containing a discharge conducting 5 gaseous atmosphere free from hydrogen and having a quartz wall portion, said device being designed ior operation with the said quartz,wall

portion at a temperature at which quartz is 1 pervious to hydrogen, said atmosphere being chemically inert to quartz at said temperature, and an alkali and alkaline earth free glaze consisting essentially oi material of the group consisting of boric oxide and phosphoric oxide on said quartz wall portion making it impervious to hydrogen at said temperature to keep the gaseous atmosphere within said envelope free from casual hydrogen in the atmosphere without said envelope.

- 6. A gaseous electric discharge device deleterim ously aflected by hydrogen in its discharge conducting gaseous atmosphere comprising a sealed envelope containing a discharge conducting gaseous atmosphere freefrom hydrogen and having a quartz wall portion, said device being designed for operation with the said quartz wall portion at a temperature at which quartz is pervious to hydrogen, said atmosphere being chemically inert to quartz at said temperature and including mercury the vapor of which is luminosity producing during operation of the device, and an alkali and alkaline earthfree glaze consisting essentially of material or the'group consisting oi' -,boric oxide and phosphoric oxide on said quartz wall portion making it impervious to hydrogen at said temperature tovkeep the gaseous atmosphere within said envelope free from casual hydrogen in the atmosphere withoutsaid envelope.

'l. A gaseous electric discharge device deleteriously aflected by'hydrogen in its discharge conducting gaseous atmosphere comprising a sealed envelope containing cooperating main discharge supporting electrodes and a discharge conducting gaseous atmosphere free from hydrogen and having a quartz wall portion, said device being designed for operation with the said quartz wall portion at a temperature at which quartz is pervious to hydrogen, said atmosphere being chemically inert to quartz at said temperature, and an alkali and alkaline earth free glaze consisting essentially of material of the group cone sisting of boric oxide and phosphoric oxide on said quartz wall portion'making it impervious to hydrogen at said temperature to keep the gaseous atmosphere within said envelope free from casual hydrogen in the atmosphere without said envelope.

8. A high pressure gaseous electric discharge lamp deleteriously affected by hydrogen in its discharge conducting gaseous atmosphere comprising a sealed vitreous jacket and a sealed envelope enclosed within said jacket and containing a discharge conducting gaseous atmosphere free from hydrogen and having a quartz wall portion, said device being designed for operation with the said quartz wall portion at a temperature at which 55 quartz is pervious to hydrogen, saidv atmosphere being chemically inert to quartz at said tempera-' ture, and an alkali and alkaline earth iree glaze consisting essentially of' material of the group consisting o; boric oxide and phosphoric oxide on said quartz wall portion making it impervious to hydrogen at said temperature to keep the gaseous atmosphere within saidenvelope free from casual hydrogen in the jacket.

9. A gaseous electric discharge device deleteri- 76 ously aflected by hydrogen in its discharge conii a ducting gaseous atmosphere comprising a pealed envelope containing a discharge conducting gaseous atmosphere free from hydrogen and having.

a quartz wall portion, said device being designed for operation with the said quartz wall portion at a temperature at which quartz is pervious to hydrogen and an alkali and alkaline earth free glaze consisting oi boric oxide on said quartz wall portion making it impervious to hydrogen at said temperature to keep the gaseous atmosphere within said envelope free from casual hydrogen in the atmosphere without said envelo e.

10. A gaseous electric discharge device deleteri- Iously aflected byhydrogen in itsdischarge conducting'gaseous atmosphere comprising a sealed envelope. containing a discharge conducting gaseous atmosphere free from hydrogen and hav-..

ing a quartz wall portion, said device being designed ior operation with said quartz wall portion at a temperature at which quartz is pervious to hydrogen, said atmosphere being chemically inert alkaline earth free glaze on said quarts wall portion' making it impervious to. hydrogen at said temperature to keep the gaseous atmosphere.

within said envelope tree from casual hydrogen in the atmosphere withoutsaid envelope. said glaze being composed of material selected fromthe group consisting of boric oxide, phosphoric oxide. silicaand alumina andcontaining at least one or the first two of said materials. a EDWARD n. NOEL. REFERENCES crrsp The following references" are of record in the file of this patent:'

to quarts at said temperature, and an alkali and x Lemmers lr' Goodwin Apr. 15, 1941 

